1. Field of the Invention
The present invention relates to a novel process for the preparation of furfural from vegetable matter and to the installation for carrying out this process.
Furfural is a compound of the formula: ##STR1## which is of great industrial importance by reason of its numerous applications, notably for the manufacture of synthetic textile fibers, of plastics materials, of synthetic rubbers, etc . . . .
Furfural is obtained from vegetable matter containing pentosans, such as ears of corn, oat, rice or cotton husks, by hydrolysis of the pentosans to obtain pentoses, which by dehydration, give furfural, in accordance with the following reactions: ##STR2##
2. Description of the Prior Art
It has been proposed to manufacture furfural from vegetable matter containing pentosans by treatment of the latter, previously moistened with a non-volatile dilute acid, which plays the role of catalyst for the degradation of the pentosans according to the reactions (1) and (2), by steam under relatively high pressure, of the order of 4.2 kg/cm.sup.2. According to this technique, described in U.S. Pat. No. 1,735,084 of Sept. 7, 1922, treatment with steam under pressure effects at the same time the degradation of the pentosans and the distillation of the furfural resulting from this degradation. However, the yields of furfural obtained by this method are very low, so that the latter only has slight economic interest for the production of furfural on an industrial scale. This is why it has been sought to improve processes for the production of furfural from vegetable matter containing pentosans, by subjecting the latter to steam treatment including essentially two steps, as described in French Pat. No. 1,181,953 of Sept. 3, 1957. According to the process described in this French Patent, the raw material is subjected to a prior treatment with steam, at a pressure close to atmospheric pressure, then to a second treatment step by steam at a higher pressure, varying from 2.8 kg/cm.sup.2 at the beginning of the operation to 7 kg/cm.sup.2 at the end of the operation. The non-volatile acid used as catalyst may be sulfuric acid, phosphoric acid or an alkane-sulfonic acid and which can be added to the vegetable matter indifferently either in the second step of the treatment, or in the prior treatment step. The steam utilized in super-heated to about 270.degree. C. at a pressure of 10.5 kg/cm.sup.2 and the yield of furfural is 68% of theory, which should correspond to about 16% by weight of furfural with respect to the dry weight of the charge. The first step of the process may proceed in a vessel of less robust construction than the digester, which must be adapted to withstand high temperatures and pressures, in which the second step of the process develops. This process however also presents numerous drawbacks, represented by:--the yields of furfural that it provides are still insufficient and--the application of the high temperatures and steam pressures, which necessitate the use, in the course at least of the second step of the process, of special equipment, generally very expensive, adapted to withstand such temperatures and pressures, and have explosion risks.
There also exists a process called "Agrifurane Process" (cf. "TECHNIQUES DE L'INGENIEUR--Genie chimique", Vol 4, page J. 6020-1501) for the production of furfural by hydrolysis in an acid medium of vegetable matter rich in pentosans. This process effects the hydrolysis by the injection of steam into steel reactors under a pressure of 10 bars. The furfuralized vapors which emerge from these reactors contain 5 to 6% by weight of furfural, so that it is necessary, to recover the technical 90% furfural, not only to condense them but to subject them to azeotropic distillation, which is a relatively complicated and expensive operation. The yield of furfural obtained is of 10 to 13% with respect to the dry weight of the raw matter utilized. This process hence has the drawback of applying high pressures, which necessitate the use of reactors resistant to these pressures, and the yield of furfural which it allows to be obtained is extremely low and can only be achieved at the cost of treatments for the removal of the water which involves relatively expensive equipment and which are long and complicated.
It is consequently an object of the present invention to provide an improved process for the production of furfural, which responds better to the necessities of practice than the processes proposed according to the prior art, notably in that it is more economical than the processes of the prior art.
It is another object of the invention to provide a process for the production of furfural which does not have to resort to the application of high temperature and pressures.
It is a further object of the invention to provide a process for the production of furfural which permits the recovery and recycling of the acid used as catalyst.
It is yet another object of the invention to provide a process which enables the yields of furfural production to be improved.
It is yet another object of the invention which provides a process which can be applied with relatively inexpensive equipment, and which does not have to undergo stress imposed by high temperatures and pressures.
Other objects and advantages of the invention will appear from the description which follows.